The main objectives are the identification and characterization of mutant mice with immunological deficiences. All experimental models will be mice maintained on various genetic backgrounds in the Mouse Mutant Stock Center Center or within individual research colonies at the Jackson Laboratory. Normal littermates will serve as controls. The immune system will be studied histologically and functionally. The effects of new immunological mutations on spontaneous tumor incidence will be investigated. By correlating specific immune deficiencies with tumor incidence and type, the relationship between immunodeficiency and cancer development might be better defined.